Removing unused method HexGrid.allPositionsInThird

[john-science]

What is the change?

Removing an unused method.

Why is the change being made?

This method has never been used by ARMI or any downstream project (except one) in the last 4-5 years. So I am moving it to the downstream project.

I find this method to be a bit too specific to have in the HexGrid class.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@opotowsky I have opened a parallel PR downstream, that you should see.

[opotowsky]

Easy :-)